The use of the magnetic properties of ferromagnetic members for sensing force related parameters has, in large part, been confined to the sensing and measurement of the torque applied to a rotating shaft. Although, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades, only in the last few years have there been breakthroughs in the design and development of inexpensive, non-contact torque sensing devices which are capable of continuous torque measurements over extended periods of time. See, e.g., U.S. Pat. No. 4,760,745--Garshelis. In addition, magnetic properties have been used for sensing tensile and compressive loads, pressure, vibration and acceleration. See, e.g., U.S. Pat. Nos. 4,802,368 and 4,875,709.
Most of the recent work in the design and development of magnetoelastic torque and other force sensors has been an outgrowth of the well known principle that the permeability of magnetic materials changes due to applied stress. For example, when a torsional stress is applied to a cylindrical shaft of magnetostrictive material, each element in the shaft is subjected to a shearing stress. This shearing stress may be expressed in terms of a tensile stress and an equal and perpendicular compressive stress with the magnitude of each stress being directly proportional to the distance between the shaft axis and the element. The directions of maximum tension and compression occur along tangents to 45.degree. left-handed and 45.degree. right-handed helices about the axis of the shaft. The effect of the torque is to increase the magnetic permeability in directions parallel to one of the helices and, correspondingly, to decrease the magnetic permeability in directions parallel to the other of the helices. As a result, the voltage induced in pickup or measuring coils surrounding the shaft increases or decreases. Inasmuch as the permeability being sensed doesn't change very much, the difference in magnitude of the induced voltages, which is proportional to the torsional stress applied to the shaft, is used as the indicator of permeability change. A similar measurement technique is used in other force sensors, wherein an applied force creates compressive and tensile stresses which alter the permeability in an appropriate ferromagnetic element to produce an electrical signal which is proportional to the applied force.
The permeability change upon which the operation of these magnetoelastic torque transducers and other force sensors is based is more specifically termed the "average permeability". To better understand what is meant by "average permeability" reference is had to FIG. 1 which illustrates a major hysteresis loop of a typical ferromagnetic material subjected to a bipolar cyclic magnetization. The hysteresis loop is a plot of applied magnetic field (H) against magnetic flux density (B) (a comparable graphical relationship would result if magnetization or intrinsic flux density (M) was plotted as the ordinate since B and M are related by the equation B=H+4M). Point A on the hysteresis loop of FIG. 1 corresponds to the peak magnetization at the maximum applied field. After the applied field has been removed such that it no longer influences the magnetization, i.e., the applied field is zero, the magnetization drops to its remanent value, corresponding to B. Likewise, point D corresponds to the peak magnetization at the maximum opposite applied field. However, after this opposite applied field has been removed such that it no longer influences the magnetization, the magnetization drops to its remanent value corresponding to E. The slope of the line DOA is the average permeability.
To better understand the significance of average permeability it is helpful to understand that the ferromagnetic hysteresis loop (FIG. 1) is probably the most fundamental expression of magnetic ordering. This complex phenomenon involves microscopic parameters such as exchange interactions, magnetocrystalline anisotropy, magnetostriction, etc., and macroscopic parameters such as domains, magnetic domain wall dynamics, magnetostatic energy, etc., and results in a process where the magnetic induction is not uniquely defined for a single value of the applied field.
In 1963, A. Globus proposed a simple model for magnetization processes in polycrystalline materials. In this model, the sample is represented by a spherical grain, divided into two 180.degree. magnetic domains by a Bloch-type magnetic domain wall, designated "B" in FIG. 2. If no external field is applied, the wall is pinned to the grain boundary in a diametrical position and no net magnetization exists. A low applied field leads to a bowing of the wall which behaves like an elastic membrane against the "pressure" of the external field. The bowing of the wall is a reversible process and accounts for the reversible magnetization range (low fields) in the magnetization (M) versus applied field (H) plot. The more horizontal portions of the hysteresis loop follow from reversible wall bowing and domain vector rotation. For higher applied fields, the wall is depinned and displaced within the grain. The more vertical portions of the hysteresis loop follow from irreversible depinning and displacement of domain wall position. The field for which the depinning occurs is the "critical" field H.sub.cr and represents the boundary between reversible (initial permeability) and irreversible (hysteresis) processes. For H&gt;H.sub.cr, there exists an equilibrium position for which the wall is again pinned. If the applied field is eliminated, the wall remains at the same position and recovers its plane shape (point B in FIG. 3). This accounts for the remanent magnetization. In order to obtain an hysteresis loop an opposite field has to be applied. At first, the wall is only bowed, until the field reaches the value of the critical field for the new position of the wall. The wall is then depinned and pushed to a new position in the other side of the grain (point D in FIG. 3). This position is again obtained for an equilibrium between the pinning force and the force due to the magnetic pressure of the applied field. By symmetry, the hysteresis loop can be constructed and the qualitative effect of an alternate field can be seen easily.
For a grain under stress all domain walls will be stiffened by the addition of stress anisotropy along some principal stress direction. One result of this is that permeability diminishes. Another result of applied stress is that it alters the effective field. This is due to the "cosine effect", i.e., as the stress anisotropy rotates the domain magnetizations to more nearly align with the applied stress, the angle .theta. between the field direction and the magnetization changes and the field becomes more or less effective according to the relationship H.sub.eff =H.sub.appl cos .theta.. As a consequence the grains will experience an increase or decrease in permeability. The average permeability is the sum of these reversible and irreversible permeability sources which, as explained, are affected oppositely by stress, except by the cosine effect. Therefore, average permeability, by its nature, effectively subtracts the reversible change from the irreversible change. The two are in opposite directions and the irreversible change is generally much larger. Thus, the percentage change of either reversible change or irreversible change, considered alone, will always be greater than the percentage change in average permeability. For this reason, average permeability is not a particularly sensitive indicator of stress.
One would be inclined to think that the very substantial effects of stress on the magnetization could be easily detected from the attendant changes in the associated magnetic fields in the space near any one domain. The key word here is "near". Since a domain does not exist in isolation, but borders on at least one other having opposite polarity, and this compensating pair is surrounded by other compensating pairs (generally having different initial orientations or mirror image polarities) it would require precisely placed field sensors, of microscopic size, to be near enough to one domain and simultaneously far enough from others having opposite polarity, to detect a stress dependent net field. The alternative to field sensors which are small enough to intercept the existing flux closure paths is to enlarge the paths sufficiently to ensure that at least some flux passes far enough outside the stressed member to be intercepted by practicable sensing means. To accomplish this the individual domain moments must be made to sum to a net bulk moment. This requires altering the balance between the volumes of domains having upwardly directed and downwardly directed moments. Material in such an unbalanced state is polarized (or magnetized as distinct from demagnetized when the volume weighted, vector sum of the internal moments is zero).
It is, therefore, apparent that despite the many advances in force sensor technology there still exists a need for a force sensor and a method for sensing force related parameters which is more sensitive to changes in stress than are known sensors and methods and which are less sensitive to temperature, magnetic fields and other ambient influences.